Melanins (eumelanins, pheomelanins) are photoprotective pigments. They remove light energy, dissipating it either as heat or in a chemical reaction in which oxygen is consumed. Recent work has shown that they also scavenge toxic species, e.g. superoxide and singlet oxygen. Protection may arise either from light filtering, removal of oxygen, or such scavenging. The cost of the protection is measured in terms of the chemical or thermal damage that may result. If the damage outweighs the protection afforded by the pigment, phototoxicity results: it has been hypothesized that damage may be more pronounced with pheomelanins than with eumelanins and may promote skin cancer in red-haired individuals. New experiments are proposed to: (i) measure the effectiveness of isolated pigments in scavenging superoxide and singlet oxygen and the resulting damage to the melanin (e.g. bleaching, irreversible free-radical production); (ii) determine whether scavenging reactions are effective in melanosomal and cellular systems; (iii) determine the effect of melanin on cell respiration and survival following exposure either to visible or UV light in the absence or presence of sensitizers. These experiments will include a comparison of eumelanins and pheomelanins, and an assessment of the utility of sensitizers in promoting chemical or thermal damage in pigmented cells will be made. In these studies use will be made of recently-developed electron spin resonance (ESR) procedures for measuring oxygen consumption and superoxide production in photolytic and cellular systems and of a new model pigmented cell system: Chinese hamster ovary (CHO) cells containing phagocytized melanin.
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